selftests/resctrl: Correct benchmark command help

Benchmark command must be the last argument because it consumes all the
remaining arguments but help misleadingly shows it as the first
argument. The benchmark command is also shown in quotes but it does not
match with the code.

Correct -b argument place in the help message and remove the quotes.
Tweak also how the options are presented by using ... notation.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Tested-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Reviewed-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: "Wieczor-Retman, Maciej" <maciej.wieczor-retman@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>